Line length and line height: what the standard says, and what the research adds

What WCAG 2.2 says

The W3C’s accessibility standard has a success criterion called Visual Presentation (1.4.8). It sits at level AAA, the highest level, so it is a stretch goal rather than the usual AA target. Among other things it asks that:

• Width is no more than 80 characters or glyphs (40 for Chinese, Japanese and Korean text).
• Text is not justified, meaning not aligned to both the left and right margins.
• Line spacing is at least space-and-a-half within paragraphs, and paragraph spacing is at least 1.5 times larger than the line spacing.
• Text can be resized to 200 percent without needing to scroll horizontally to read a line.

The W3C’s explanation says long lines can become a significant barrier for people with some reading or vision disabilities, and that people with some cognitive disabilities find it hard to track text when the lines are close together. The 40-character limit for CJK text comes from studies the W3C cites, which found those characters to be about twice as wide as non-CJK characters.

A separate AA criterion, Resize Text (1.4.4), asks that text can be enlarged up to 200 percent without losing content or function. It is about what a reader can do with your page, not about a minimum font size.

Line height in practice

MDN, the web documentation project, recommends a minimum line-height of 1.5 for main paragraph content, to help people with low vision and cognitive concerns such as dyslexia. It also prefers a unitless number (1.5, not 1.5em), because a unitless value is multiplied by each element’s own font size and keeps scaling when a reader zooms. A value in em units is computed once and inherited, which can give unexpected results.

Line length: where the evidence is mixed

Eighty characters is a ceiling, not a target, and the research does not point to one best width.

In a study of reading from screen (Dyson and Kipping, in the journal Visible Language), readers read 100-character lines faster than very short 25-character lines, with no loss of comprehension. The same readers’ opinions went the other way: they rated the longest lines hardest to read and moderate lines, around 55 characters, easiest. In other words, what people say they prefer and how fast they read did not match.

Two cautions. We read the abstract, not the full paper, and the study is nearly three decades old, so we would not build a rule on it alone. What it does support is a plain point: do not assume the narrowest column is the fastest or the easiest. The W3C ceiling and the preference result together suggest a moderate width, but no source we read names an exact best number.

What a store owner can do this week

1. Find your longest text blocks: product descriptions, blog posts, policy pages. On a desktop screen, count the characters in one full line. If it is well over 80, narrow the text column rather than making the type smaller.
2. Check the line height of body text in your theme. If it is below 1.5, raise it and look at the page on a phone as well as a desktop.
3. Check that body text is left-aligned, not justified.
4. Zoom the browser to 200 percent and confirm nothing is cut off and you do not have to scroll sideways to read a line.
5. Set widths with relative measurements so lines stay at or under 80 characters as the window changes; the W3C lists this as a technique (C20).